Fiber-optic transmission networks provide transmission for multiple channels using wavelength division multiplexing (WDM). Optical amplifiers, such as erbium-doped fiber amplifiers (EDFAs), provide a mechanism for boosting power after the multiple channels are attenuated due to fiber loss from being transmitted over a distance. Additionally, fiber-optic transmission networks can include other components, such as variable optical attenuators (VOAs), wavelength selective switches (WSSs), wavelength blockers, and the like, which in addition to other functionality can adjust per-channel power (or per group of channels).
Referring to FIG. 1, an exemplary reconfigurable optical network 10 is illustrated showing a single transmission path. Fiber-optic networks typically include both a transmit and receive path for bidirectional communication, but FIG. 1 depicts only a unidirectional path for simpler illustration purposes. Topologies of fiber-optic transmission networks include ring, linear, mesh, and combinations thereof. FIG. 1 illustrates a ring topology with a linear spur through a reconfigurable optical add-drop multiplexer (ROADM) 30. The network 10 is shown with four nodes 20 each including a pre-amplifier 22 located at the input to the nodes 20 and a post-amplifier 28 located at the output of the nodes 20. The pre-amplifier 22 provides optical amplification prior to de-multiplexing of channels (i.e., wavelengths), and the post-amplifier 28 provides optical amplification after multiplexing of channels prior to transmission on a fiber 14.
Each of the nodes 20 illustrated in FIG. 1 include a mid-stage point between the pre- and post-amplifiers 22,28 including a fixed optical add-drop multiplexer (OADM) 24, a variable optical attenuator (VOA) 26, and the ROADM 30. Each of the components 24,26,30 is capable of providing per-channel or per group of channel attenuation on wavelengths that are added, dropped, or expressed through the node. The OADM 24 provides for adding and dropping of fixed wavelengths, and the ROADM 30 provides for reconfigurable add/drop of any wavelength. ROADMs 30 can include a micro-electromechanical system (MEMS)-based wavelength-selective switch (WSS), a wavelength blocker, a multiplexer, and the like.
Each node 20 in the reconfigurable optical network 10 simultaneously receives optical channels from other nodes 20 over the optical fiber 14. The optical paths of the optically-multiplexed received channels may either be terminated (“dropped”) at the current node 20 with the ROADM 30 or OADM 24, or switched to continue on to other nodes (“expressed”) over additional optical fiber connections with the ROADM 30, OADM 24, or VOA 26. Such path routing may be accomplished by a number of technologies, including the WSS. The WSS provides adjustment of the individual channel attenuations, so that, for example, power equalization may be effected. When combined with the wideband optical amplifiers 22,28 at the node 20, the WSS also contributes to control of the net gains for each individual channel at the node. The optical amplifier 22,28, through a single gain setting, affects all of the channels at once.
The stability and setting accuracy of optical powers within reconfigurable optical networks 10 is a universal goal of equipment providers and network operators. The amplifiers 22,28 and components 24,26,30 each are adjusted to account for different power levels based on a variety of factors, such as channel count. A common obstacle in meeting the stability and setting accuracy goal is the change that occurs in optical gain for channels that are active after a system event. Such an event can include unplanned (e.g., through equipment failure or fiber cut) or planned (e.g., addition or deletion of channels) changes in the number of channels passing through the amplifiers.
Optical power-control-loops are designed to counter individual or collective power changes and to converge signal powers (collectively and individually) to target values. For example, these optical power-control-loops are configured to dynamically provide attenuation to individual channels through the various components 24,26,30, and to adjust the overall gain of the amplifiers 22,28. The speed of convergence of signal powers to targets is higher when the express-channel node-gain is controlled independently of channel output-power. An example of such power-control-loops is described in commonly-assigned U.S. patent application Ser. No. 11/786,143, filed Apr. 10, 2007, and entitled “METHODS AND SYSTEMS TO STABILIZE AN OPTICAL NETWORK AGAINST NODAL GAIN CHANGES.”
Amplifiers 22,28 and per-channel attenuation devices have defined limits to their operating gains and losses. Furthermore, optimal signal-to-noise ratio (OSNR) depends on the relative settings of amplifier 22,28 gain, which affects all channels together, and per-channel attenuation, which affects channels individually. For example, in the case where optical amplification occurs before a WSS, ideal channel performance is obtained when the amplifiers are operated at the highest gain possible.
Currently, methods independently control amplifiers 22,28 to gain or power targets, and set per-channel attenuations to achieve separately-determined attenuation targets for individual channels. Alternatively, current methods rapidly determine that limits have been reached on attenuation or amplifier control, and use a slower, external, control process (control loop) to optimally determine the division between amplifier gain and per-channel attenuation loss.
Ideally, a single controller would be used to control both amplifier gain and attenuation. In addition, the use of an industry-standard proportional-integral-differential (PID) control process is desirable. However, PID control is not typically applicable to multiple-input multiple-output (MIMO) systems. The nodes 20 in the reconfigurable optical network 10 are MIMO systems with the multiple inputs and outputs including the channels dropped and expressed through the node 20.
Separate control of amplifier and attenuation settings makes it difficult to maximize signal-to-noise ratio. Use of a separate control loop to arbitrate between amplifier gain and per-channel attenuation must be at least ten times slower than the amplifier and WSS control loops in order to maintain process stability. This results in longer periods of time when non-optimal signal-to-noise ratio is in effect.
Referring to FIG. 2, a proportional-integral-derivative controller (PID controller) 40 is a common feedback loop component in control systems. The PID controller 40 takes a measured value from a process or other apparatus and compares it with a reference setpoint value (i.e., summation block 41). The difference (or “error” signal) is then used to adjust some input to the process in order to bring the process' measured value to its desired setpoint. Unlike simpler controllers, the PID can adjust process outputs based on the history and rate of change of the error signal, which gives more accurate and stable control. In contrast to more complex algorithms such as optimal control theory, PID controllers 40 can often be adjusted without advanced mathematics. For example, the PID controller 40 can be implemented quickly and efficiently in a microprocessor or a digital signal processor (DSP).
The PID controller 40 includes three terms including a proportional term 42, an integral term 43, and a derivative term 44. The error signal from the summation block 41 is input to each of the terms 42,43,44. The proportional term 42 handles the immediate error, the error is multiplied by a constant Kp. Note that when the error is zero, the proportional term 42 is zero. The constant Kp is the proportional gain and the larger Kp typically means faster response since the larger the error, the larger the feedback to compensate. The integral term 42 enables the controller 40 to learn from the past by integrating the error and multiplying by a constant Ki. The integral term 42 allows the controller 40 to eliminate a steady state error if the process requires a non-zero input to produce the desired setpoint.
The integral term 42 will react to the error by accumulating a value that is added to the output value. While this will force the controller to approach the setpoint quicker than the proportional term 42 alone and eliminate steady state error, it also guarantees that the process will overshoot the setpoint since the integral value will continue to be added to the output value. The constant Ki is the integral time and the smaller Ki implies steady state errors are eliminated quicker with the tradeoff being a larger overshoot, i.e. any negative error integrated during transient response must be integrated away by positive error before we reach steady state.
The derivative term 44 allows the controller 40 to anticipate the future by taking the first derivative of the error and multiplying it by a constant Kd. This can be used to reduce the magnitude of the overshoot produced by the integral component, but the controller will be a bit slower to reach the setpoint initially. The constant Kd is the derivative time with a larger Kd decreasing overshoot, but slowing down transient response. There are several tuning algorithms for the PID controller which set the constant values.
Disadvantageously, MIMO systems are complex to control and PID control cannot be applied to a reconfigurable optical node because it is a MIMO system.